Thermionic tube



G. W. PICKARD THERMIONIC TUBE Filed March '7, 1922 2 Sheets-Sheet 1 INVEN TOR ArmRNEY G. W., PICKARD THERMIONIG TUBE Filed March 7, 1922 n 2Sheets-Sheet 2 PLB 341' @L5 y fi,

IN VEN TOR lA TTORNE Y Patented Nov. 22, 192.7.

UNITED STA-TES PATENT OFFICE.

GREENLEAF WHITTIER PICKARD, OF NEWTON CENTER, MASSACHUSETTS, ASBIGNOR T0WIRELESS SPECIALTY APPARATUS CQHPANY, Ol' BOSTON, MASSACHUSETTS, A

CORPORATION OF NEW YORK.

THEBHIONIC TUBE.

Application led Harsh 7, 1922. Serial N0. 541,603.

This invention relates to thermionic tubes.

The object of the invention is to provide for such tubes an emitter ofelectrons which will permit the employment of alternating current forheating while at the same time prevent the varying potential of suchcurrent from deleteriously alecting the action of the elements of thetube.

' The invention consists of an organization substantially as describedherein, pointed out as to novelt-y in the claims, and shown in theaccompanying drawings which are about four times scale, and of whichFigure 1 is alongitudinal section;

Fig. 2 isa like view of a modification; Fig. 3 is'a likerview of anothermodification;

Fig. 4.- a like view of yet another modification; and

Fig. 5 a diagrammatic illustration of a radio receiver set employing theinvention in each of a plurality of tubes.

Heretofore tube-filaments 0r emitters for receiving tubes have beenheated by direct current as from storage batteries, this notwithstandingthe desirability for convenience of employing alternating current, andon account of the disturbance or hum created by the potential across thelegs of the filament if an alternating current be used. It is highlydesirable to employ alternating current because it may be easily andcheaply transformed down to the proper voltage for the filament, and isnearly always available, as in practically all house-lighting circuits.The employment of storage batteries is much less desirable because theyinvolve an initial expense constituting a large part of the total costof radio-receiving apparatus; they must be frequently re'charged,necessitating either the use of an adjacent rectifier, or thetransportation of the battery to a distant charging station; and thisrecharging becomes a greater inconvenience when an amplifier trainpresent-day practice, which makes a large demand upon the battery. Alsothe storage battery is a source ot acid spray. But notwithstanding thedisadvantages of the storage battery and the advantages of the,alternating current, use of the battery with relv ceiving tubes hascontinued up to the present time on account ofthe etfect Vof thealternating difference of potentlal across the legs of several tubes isused as in of the filament when alternatn current 1a employed. Thisvarying potential creates a. varying electrostatic field in the vicinityof the filament which induces a varying potential on the grid or controlelement of the tube, thereby varying the plate-filament current so as tocause a loud hum in the telephone receivers supplied by the tube. Thisaction of the alternating current is probably somewhat complex, but maybe assumed to be due in large part to the var ing potential induced onthe grid; secon arily, to the varying electrostatic eld around thefilament itself; and in some part also due to thermal variations of thefilament, at least when a thin filament is used. While alternatingcurrent for heating the filament has been used in high power tubes forradio transmitter use (wherein the effect of the alternating fieldsaround the lilament is swamped by the high potentials employed on thegrid and plate elements of the tube), yet in tubes employed for radioreceiving purposes, the direct current as from stora e batteries hasbeen universally employed y reason of the above.

In accordance with this invention, the source of the emission ofelectrons for use in the operation of the tube is not the incandescentilament, as generally employed heretofore, although there is employed anelectrical heater adapted for operation by an alternating current. Herethe emitter of electrons consists of an element the surface of which isat substantially the same potential at all points and times. Also theemitting elementmay serve not only as a source of electrons but vas ashield located between the heating element and the other elements of thetube such as the grid and plate.

In Fig. 1, the emit-ter for glass tube T is shown as comprising a thinplatinum tube or thimble F, permissively about one-sixteenth of an inchor less in outside diameter and, say, about one-quarteror one-fifth ofan inch long. I In all the drawings, the emitter is shown in enlargedscale, and actually these arts will be made as small as possible in orer to avoid the use of a large heater for the emitter, and in order tokeep down the electrostatic-capacity of the tube (i. e., the capacitybetween the grid and emitter and the capacity between the grid andplate), so that the'tube may be used eifec-v tivel on shortwave-lengths. As to the desire small size of the heater, it is to benoted that the heat dissipated at a given temperature is directlyproportional to the area. An emitter of the abo-ve general d1- mensionsmay have an input of, say, 2.8 Watts and an electron-emitting area of,say, 0.3 square centimeter.

The outside surface of emitter cylinder F is coated with lime or othersuitable oxid, such as barium or strontium, in the manner now Well knownin the art (a V or hairpin filament has been used, coated with some suchoxid, for the purpose ot' emitting electrons in abundance in atliermionic tube; but like the other filaments employed, .this could notbe used with alternating current for the reasons above stated).

Inside of tube F is a heater H, consist-ing of a fine wire hairpin orcoil of suitable material which will stand the. desiredteniperatureWithout melting, i. e., such as nichroine, tungsten and the like.Surrouinlingl heater I-I and filling the interior of tube F may be asuitable insulating lilling I such as a high melting point enamel ormica. Insulator I may be air, or any substance which is a reasonablygood conductor of heat, with a meltin point suiiiciently high toWitlistand a red ieat, and a fair electrical insulator for low voltages.Also it should be of some substance, as stated, which will not interferewith the exhaustion of the therinionie tube or with the maintenance ofthe high vacuum during the life of the tube. (By air, I mean the gaseousfluid, whatever it may be which is within the glass tube and which ismaintained at a low pressure usually referred to as a more or lessperfect vacuum.) Crushed quartz may be used for this insulating fillingI, packed or taniped into tube F so as to hold heater H securely incentral position.

The grid element and the plate element P may be arranged in any usualmanner as Shown, i. e., concentric with emitter F.

The entire emitter F, with its heater I-I, is supported on wires W ofplatinum or. the like, which are thin to prevent thermal conduction awayfrom the emitter. One of these supporting wires W is electricallyconnected by conductor X with au electrostatic shield S which surroundsthe entrance leads A, A to heater H. (But shield S may be electricallyconnected with emitter F in any way which may be 'found convenient inthe assembly of the thermionic tube.) This shield S extends up near tothe lower edge of thiinble F. but does not touch it, being thereinsulated from it. Shield S may be of any thin construction of metalsuch as nickeL-suitably supported in relation to glass tube T, and,together with thinible emitter F, shield S forms anelectrically-continuous surface all permissively at the saine potentialthrough- .not surround the leads in the sense of causing resultan'tdeleterious action from alternating currents through the leads.

The various leads from tube T may be taken out in any desired manner, asusual; the heating leads A, A being taken out through glass tube T asshown, from heater H, the lead G1 from grid G, and the lead P1 fromplate P. The lead from emitter F (the outside circuit lead asdistinguished from the heating Wires A, A) may be W1 as a continuationof one of the supporting Wires W.

The heating wires A, A may be connected to any suitable source ofalternating current of the proper potential, such as the secondary Z(Fig. l) of an ordinary alternating current transformer, the primary ofwhich, not shown, may be connected to Vthe ordinary house-lightingmains, as atv 60 cycles.

Emitting tube F may consist of platinum, alloyed with a small percentageof iridium (6%), and covered with the oxids of bariuin and strontium.These oXids may be applied alternately, andafter each oxid applicationthe temperature of the tube is momentarily raised to about 1,0000 C. Asmany as sixteen such applications may be made, and after that the tubemay be baked 'at about 1,200o C. for two hours. Heater H preferably inaybe so designed that when operated on the Working alternating currentvoltage, there will be dissipated in the tube approximately eight wattsper square centimeter of tube F aiea. The area of the tube F will, of

course, depend upon the desired plate-current, and for ordinary detectorand amplifier work this area may be generally between 0.2 and 0.5 squarecentimeter, so that the heater will be designed to dissipate between.1.6 and 4.0 watts.

Such a tube as the above is adapted for heating of the emitter byalternating current, for any purpose where the potentials of the plateand grid are not sufliciently high to swamp the alternating fieldsaround the filament. For example, the tube of this invention may be usedin radio communication receiving circuits as detector or amplilier (seeFig. 5), and it may be used as a relay in telephone lilies, astheiniionic tubes now are employed: and in all such cases the step-downalternating current transformer Z (Fig. 1) or any other convenientsource 0f alternating current may be substituted for the storage batteryheretofore used with prior thermionic tubes, without any deletenouseffect on the thermionic tube of this 1nvention or its operation.

The invention lnay be embodied in various forms, but its embodimentrequires nothing out ot' the usual practice in respect of the design orexhaustion of the glass or other structure which may enclose theelements. Those skilled in the art of thermionic tubes may employ incombination any or all of the things advantageous for such tubes, inembodiments of the invention which may be specifically different fromany form shown herein. I

The emitter F, in that it may consist of a metal base having the oxidcoating, is to that extent like the lVehnelt cathode construction whichheretofore has been employed in the form of V or hairpin filaments inthermionic tubes, where the oXid-coated filament was heated by directcurrent or from a storage battery; but the emitter hereof may consistequivalently of any material or materials capable of emitting electronsfreely when heated, high incandescence not being requisite in this classof thermionic emitters as in the case of uncoated filament `the tube, inany case.

emitters.

In the modification of Fig. .2, heater H1 (instead of being a hairpinfilament insulated from emitter F), is a fine wire electricallyconnected to the upper end of emitter F whereby the latter acts as; areturn heating lead, thereby simplifying the construction. In Fig. 2 theemitter F is in the form of Fig. 1 with its exterior o Xid layer FO. Thecross section of this emitter F is so proportioned relative to thatl ofits heater H1 that the voltage drop along the emitter willbe little, ifany, and substantially negligible inits effect on the action of Thisproportioning will result substantially without special design, whenheater I-I1 consists of a fine wire having a cross section from say tento one hundred times smaller than that of emitter F; but thisproportioning is important and -should be taken into cons1deration inthe design of emitter and heater. In this form (and in other forms asshown in'Fig. 3), no insulating substance need be used in the space-between the heating portion H1 and the emitter portion F; so that theheat is transferred to emitter F partly by radiation across the shortvacuous space between the heating and lemitting parts, and partly byconduction via Also in the mionic tube T around the heater leads HL, HLfrom any electrostatic field from said leads; and conductor X connectsshield S to lead FL. The grid coil G, cylindrical plate P and theirrespective leads GL and PL are made and brought out in the usual manner,and in Fig. 2, as stated above, one of the heater-leads HL is also theemitterlead FL to the tube-circuit. In Fig. 3 emitter F2 (having oxidcoatin FO) is in the form of a. flat circular disk o platinum F (cf.Fig. 1), about one-quarter inch in diameter or less, and say two orthree thousandths of an inch thick. This disk F is supported on acentral ri id stalk FL2, forming a part of the lead rom theA emitter tothe circuit of the tube.

Underneath disk F2 is the heater H2, H2, consisting of tun ten or otherwire of high melting point. eater vH2 and disk F2 may contact at theirmidpoints, as shown. The heater may be supplied with alternating currentby leads HL2, HL2. This heater heats the disk F2 partly by radiation andpartly by conduction at the point of contact or attachment. i Themosteffective form probably is one wherein a. thin layer of enamel thermallyconnects the coil of resistance wire H2 to'emit-ter F2, pursuant toconventional rheostat practice; thereby the enamel layer serves tosupport .the resistance wire close to the emitter to be heated,electricall sulates it therefrom, and as a thin ayer constitutes a.sufiiciently good thermal con- 1ductor to permit the effective transferof eat.

Above disk F2 the grid G2vis held in posi-4 tion by the stiff grid-leadGL2; grid G2 consistino of a flat spiral of fine wire.. Above grid 2 theplate element P2 is supported on the 'plate-lead PL2 and also, ifdesired, by a separate support PS. A nickel shield S2 electricallyshields the interior of the tube from heater leads HL, HL and iselectrically connected by conductor X2 to emitter disk F2. By the aboveconstruction and arrangement of emitter and heater, the

emitter itself will have little, if any, voltage drop along it.

In the modification of Fig. 4, the heater wire H3 is embedded in atubular insulator I3 (cf. Fig. 1), which may be a short piece areelectrically connected to emitter F3 by fine wires X3, X5, thisarrangement preventing excessive loss of heat by a thermal connection otlarge cross-section. Grid Ga and its lead GIE, and plate Ia and its leadPL3 are as in Figs. 1 and 2. Emitter lead FL3 to the external tubecircuit is taken from one of the two shields S3.

In any case, the thermionic tube may be constructed so that the emittermay be heated by current from a storage battery alternatively withalternating currents, if and whenever convenient or desired.

Fig. 5 illustrates an ordinary radio receiving set employing twothermionic tubes, each of which embodies the invention. Here, thesignals are received by antenna A, passed by transformer ()'l` to thefirst thermionie tube 'l`1 acting (at left) as detector, and then passedby the usual transformer AF to the second thermionic tube T2 acting (atright) as amplifier, whence the amplified currents pass to the telephonereceiver shown. The current for heating the heaters for the emitters F1,F2 ot the two tubes is furnished from a step-down transformer T suppliedfrom an ordinary sixty-cycle main to the heater leads A, A. The plateelement P of the first or detector tube T1 is connected via lead PLthrough transformer AF to the positive terminal of the plate battery PB(which may be a simple dry cell or cells), the negative pole of thisbattery being connected to emitter F1 of the first or detector tube T1via lead IVI. ln the two tubes here shown, each emitter, in-

cluding its heater, is indicated as being constructed, for example, likethe corresponding-parts in Fig. 1. The second or righthand tube T2,being the amplifier tube, has its grid Grl connected via lead GL to `oneterminal of the secondary of transformer AF, the other terminal of AFbeing connected to the negative terminal of battery PB. The plate P1 ofthe second tube is connected to the receiving telephones and throughthem to the positive terminal of plate battery PB. The operation of thissystem of Fig. 5 is identical with that of the ordinarydetector-amplifier train of tubes, with the exception of the emitterconstruction which-permits the use of alternating current for heating.

The forms of Figs. 2-4 may be constructed according to the preferredelectrical and thermal proportions above described for the form of Fig.1, especially as to emitter' F.

I claim 1. In an electrically heated electron emitter for low powerthermionic tubes of the evacuated glass-enclosed type having grid andplate electrodes independent of the emitter, the improvement comprisinga small emitter member having an external emitting surface and aninternal electric heater, the emission surface constituting a shieldbetween the heater and the grid of the thermionic tube; electricinsulatin material between the internal heater an the external emittingsurface and supporting the emitting surface upon the heater; and circuitleads from the heater and ex tending through the glass tube; saidemitting surface and electric heater being thermally isolated within thetube save for said circuit leads.

2. In an electrically heated electron emitter for low power thermionicvtubes of the evacuated glass-enclosed type having grid and plateelectrodes independent of the emitter, the improvement comprising asmall emitter member having an external electron-emitting surface and aninternal electric heating means; electric insulating material betweenthe internal heating means and the emitting surface and supporting theemitter upon the heating means; and circuit leads for the heating meansextending through the glass tube; said heating means and emitter beingwholly supported by said circuit leads and thermally isolated within thetube save for such leads, the latter heilig substantially thin toprevent excessive thermal conduction.

3. In a low power thermionic tube of the evacuated glass-enclosed typehaving grid and plate electrodes independent of the the improvedemitting means which comprises a small emitter member having an externalemitting surface and an internal electric heating means, said externalsurface constituting an electrode shield between the grid and theheater; circuit leads for the electric heater; and a shield of metallocated between the'grid and I portions of said leads extending outsidebeyond the emitter; said metal shield being connected by an electriclead to the emitting member but otherwise metallicallyT independentthereof.

4. In a low power thermionic tube of the evacuated glass-enclosed typehaving three independent electrodes including grid, plate andelcctron-emitter, the improved emitter which comprises a small emittermember having an external emitting surface and a neighboring electricheater. said emission surface constituting a shield between the heaterand the grid: circuitv leads J[or the heater; a shield ot' metal locatedbetween the grid and parts of said leads which extend beyond the emittermember: and electric insulating material between the internal heatingmeans and the emitter member and supporting thelatter: the emittermember and the electric heater being thermally isolated within the glasstube save for said circuit leads.

5. In a low power thermionic tube of the evacuated glass-enclosed typehaving three and electron-emitter, the improved emitter which comprisesa small emitter member having an electron-emission surface, and aneighboring electric heater therefor, said emitter member constituting ashield between the electric heater and the grid; circuit leads for theheater and extending through and supported in the glass of the tube; ashield of metal located between the grid and parts of said leads whichextend beyond the emitter' member; and electric insulating materialbetween the emitter member and the electric heater and supporting theemitter member on the heater; said metal shield being connected by anelectric lead to said emitter member butotherwise metallicallyindependent thereof:

6. In a low power thermionictube of the evacuated glass-enclosed typehavin three independent electrodes including gri plate andelectron-emitter, the improved emitter which comprises a small emittermember having an electron-emitting surface and va neighboring electricheater, said emitter member constituting a shield between the electricheater and the grid.; circuit leads for the heating means and extendingthrough and supported in the glass of the tube; a shield of metallocated between the grid and parts of said leads .which extend beyondthe emitter member, said metal shield being connected by an electriclead to said emitter member but otherwise metallically independentthereof; and said emitter member and electric heater being supportedinsidethe glass tube by said circuit leads alone independent of saidmeta-l shield.

7. In a low power thermionic tube of the evacuated glass-enclosed typehaving three independent electrodes including grid, plate andelectron-emitter, the improved construction and arrangement of emitterwhich comprises a small emitter member having an electron-emittingsurface and a neighboring electric heater, said emitter memberconstituting a shield between the grid and the heater; circuit leads forthe heater and extending through and supported in the glass of the tube;a shield of metal supported on the glass tube and located between thegrid and parts of said leads which extend bey yond the emitter member;and electric insulating material located between the emitter member andthe heater and supportin the emitter member on the heater; sai metalshield being connected by an electric lead to said emitter member butotherwise metallically independent thereof; and said emitter member andheater being supported inside the glass tube by said circuit leadsindependently of said metal shield.

GREENLEAF WHITTIER PICKARD.

